CHICAGO (Reuters) - People who are underweight have a 40 percent higher risk of dying in the first month after surgery than patients who are overweight, according to new research released on Monday.

The findings suggest that body mass index, or BMI, may be useful in predicting which patients are at the greatest risk while recovering from surgery, U.S. researchers reported in the Archives of Surgery.

Prior studies looking at the role of BMI in surgery have been mixed, said George Stukenborg of the University of Virginia in Charlottesville, who worked on the study.

Patients with low BMI are at higher risk of death 30 days after surgery, Stukenborg said in a telephone interview.

The researchers used data on nearly 190,000 patients who underwent a variety of surgeries at 183 hospitals between 2005 and 2006.

BMI is calculated by dividing weight in kilograms by height in meters squared. According to the U.S. Centers for Disease Control and Prevention, people with a BMI of 18.5 to 24.9 are normal weight, those with a BMI of 25 to 29.9 are overweight, and those with a BMI of 30 and above are obese.

To look for a link between body weight and the risk of death, they classified patients into five groups or quintiles: people with a BMI of less than 23.1; people with a BMI of 23.1 to less than 26.3; people with a BMI of 26.3 to less than 29.7; people with a BMI of 29.7 to less than 35.3; and people with a BMI of 35.3 or higher.

Overall, 2,245 or 1.7 percent of people in the study died within 30 days of surgery.

We found patients in the lowest quintile had a 40 percent higher odds of death compared to the mid-range, said Stukenborg, referring to people in the overweight category with BMIs of 26.3 to 29.7.

Even when the researchers adjusted for type of surgery and other risk factors, those with a low BMI still had a greater risk of dying in the first month after surgery compared with plumper surgery patients.

Stukenborg said it is not clear why. The study did not track recent weight loss, so it could be that people who weighed less were sicker to begin with.

That is a possibility, he said.

Either way, Stukenborg said doctors should consider BMI when they plan surgeries for their patients.

Being overweight or obese carries many other risk, raising chances of heart disease, diabetes, some cancers, arthritis and other conditions. Obesity-related diseases account for nearly 10 percent of medical spending in the United States or an estimated $147 billion a year.

(Editing by Vicki Allen)